Start mission queue check gets stuck if two requests received at the same time

[andchiind]

Describe the bug
If two requests to start a mission are received at the same time then the first will succeed but the second one will be trying to wait for a response on the queue that will never arrive.

To Reproduce
Run two missions within a couple of milliseconds.

Expected behavior
Thread number two should detect that an attempt to start a mission has already been made. This can be done in several ways, but the important thing is that a thread safe object should represent if the statemachine is trying to initiate a mission. In the worst case, if the queue times out then we can at least return a 408 instead of an internal server error.

Screenshots

[andchiind]

A solution could be to put the response on a different queue than the one we use for start_mission. Then we could put several start mission requests on the queue and ISAR could handle them in order, and respond in order.